The Use of Radioactive Aerosols to Assess Pulmonary Ventilation

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Abstract
Dry submicronic particles penetrate well to the lung periphery. They can serve as good substitutes for radioactive gases when assessing lung function or, in patients with suspected pulmonary embolism (PE), when comparing regional ventilation with perfusion. Basic jet nebulizer systems however generally give particles of too high a size for good peripheral visualization and are inefficient at delivering material to the lungs. Modifying techniques – aerosol chamber systems, settling bags, baffles etc. – have been introduced to reduce particle size. Additionally drying devices have been applied to the nebulizer air–supply and some centres have employed presized particles. Modified techniques have aimed to increase efficiency of delivery to the lungs. The present paper describes validation data on two such systems which, respectively, deliver: 0.46 μn monodisperse 99m Tc– human albumin millimicrospheres (HAmM) and 0.37 μm (GSD 1.8) "dry" 99m Tc–DTPA particles (low humidity jet nebulization of 0.09% NaCl–DTPA solution into a settling bag). Penetration index values (inner to peripheral zone ratios) for particles correlated well with those for 81m krypton gas (HAmM: n = 48, r = 0.89, p < 0.001; DTPA: n = 14, r = 0.75, p <0.001). In a comparative study on seven patients, the DTPA device gave higher deposition efficiency (mean percentage of initial equipment load deposited in the lungs: HAmM = 24%; DTPA = 36%). Larger (1.0 μm) DTPA particles generated from more concentrated (0.9%) NaCl–DTPA solution gave poorer deposition efficiency and less peripheral penetration than particles from the 0.09% solution. Satisfactory post– perfusion ventilation imaging can be obtained either by different isotope labelling (111 In, 113m In) of the aerosol or by employing different tracer doses for perfusion and ventilation imaging.